Implantable cardiac devices are well known in the art. They may take the form of implantable defibrillators or cardioverters which treat accelerated rhythms of the heart such as fibrillation. They may also take the form of implantable pacemakers which maintain the heart rate above a prescribed limit, such as, for example, to treat a bradycardia. Implantable cardiac devices are also known which incorporate both a pacemaker and a defibrillator.
A pacemaker is comprised of two major components. One component is a pulse generator which generates the pacing stimulation pulses and includes the electronic circuitry and the power cell or battery. The other component is the lead, or leads, which electrically couple the pacemaker to the heart.
Pacemakers deliver pacing pulses to the heart to cause the stimulated heart chamber to contract when the patient's own intrinsic rhythm fails. To this end, pacemakers include sensing circuits that sense cardiac activity for the detection of intrinsic cardiac events such as intrinsic atrial events (P waves) and intrinsic ventricular events (R waves). By monitoring such P waves and/or R waves, the pacemaker circuits are able to determine the intrinsic rhythm of the heart and provide stimulation pacing pulses that force atrial and/or ventricular depolarizations at appropriate times in the cardiac cycle when required to help stabilize the electrical rhythm of the heart.
Pacemakers are described as single-chamber or dual-chamber systems. A single-chamber system stimulates and senses the same chamber of the heart (atrium or ventricle). A dual-chamber system stimulates and/or senses in both chambers of the heart (atrium and ventricle). Dual-chamber systems may typically be programmed to operate in either a dual-chamber mode or a single-chamber mode. Further, pacing systems are known which pace in multiple sites. For example, biventricular pacing paces in both ventricles and biatrial pacing paces in both atria. Hence, it is possible, that a heart may be paced in all four of its chambers.
A popular mode of operation for dual-chamber pacemakers is the DDD mode. Specifically, DDD systems provide atrial pacing during atrial bradycardia, ventricle pacing during ventricular bradycardia, and atrial and ventricular pacing during combined atrial and ventricular bradycardia or heart block also known as AV block. In addition, DDDR systems monitor patient activity levels for controlling pacing rate to more closely approximate the normal response of the heart to exercise, or other physiological activity demanding a faster heart rate.
A recent study has indicated, however, that ventricular pacing in the setting of intact AV nodal conduction has an adverse impact compared to permitting intrinsic ventricular contractions. Hence, pacing therapies have been advanced which encourage intrinsic ventricular activity.
One such system employs an auto intrinsic conduction search (AICS) wherein the pacemaker utilizes two AV intervals. The first interval is a programmable base AV interval to support ventricular demand pacing. It may be, for example, on the order of two hundred (200) milliseconds. The second AV interval is an extended AV interval which may be thought of as comprising the base AV interval with an AV interval extension added to its end. The AV interval extension may be on the order of one hundred (100) milliseconds, for example. Hence, in this example, the extended AV interval would total three hundred (300) milliseconds.
The AICS may be implemented as follows. The device paces in a demand mode with the base AV interval. After a search interval of five minutes, for example, the device extends the AV interval to the extended AV interval for one cycle to encourage intrinsic ventricular activity. The device does not reset to the shorter base AV interval until a ventricular pacing pulse is administered.
Other methods to promote or encourage intrinsic conduction have been advanced. One proposed method is based upon mode switching from a DDD(R) mode to an AAI(R) to more aggressively promote intrinsic conduction. Unfortunately, this concept has disadvantages. Methods which switch to an atrial pacing mode like AAIR can create the loss of one or more ventricular contractions before the mode is switched back to a dual-chamber mode. The loss of the ventricular contraction might create symptoms which patients might find difficult tolerate. Depending on the criteria used to switch back to the dual-chamber mode, a hemodynamically compromising situation (with a too slow intrinsic conduction) can result for a longer period of time in the atrial pacing (AAIR) mode.
Another proposed method is to provide prolonged AV interval. A prolonged AV interval reduces the available programmable values for the Maximum Tracking Rate (MTR), if the post ventricular atrial refractory period (PVARP) is not shortened accordingly and a late ventricular pacing pulse that occurs when the long AV interval times-out without a sensed R-wave has a higher risk of creating a retrograde P-wave which could initiate a pacemaker mediated tachycardia (PMT). Thus, PVARP should be extended with a prolonged AV-hysteresis.
Hence, the need remains to more aggressively and effectively promote intrinsic conduction without the disadvantages of limiting the maximum tracking rate, not maintaining ventricular pacing support or creating longer periods of hemodynamic compromise. The present invention addresses these and other issues.